Thermocouple, particularly for electro-thermic cooling, and method of producing it



June 16, 1964 U. BIRKHOLZ 3,137,593

THERMOCOUPLE, PARTICULARLY FOR ELECTROTHERMIC COOLING, AND METHOD OF PRODUCING IT Filed April 23, 1959 ll l/2 a l' 3 E g 1% Se 0,5%Se (um) m (alms) 15o log 0' F |g.|

,izs 77 I i 06/ s )(m Il {asne ze I 2,5 A 0,5%Se l (aum) 2z.

' lfl; I x 2.3 I

0 Se l (u.=11.6) qk 2.1

l l 2'0 50o 79o 1000 1500 2000" United States Patent 'IHERMGCOUPLE PARTECULARLY FR ELEC- TR-THERMIC COOLING, AND ll/El-IGD F PRODUCNG IT Ulrich Birkholz, Nrnberg, Germany, assigner to Siemens- Schuckertwerke Aktiengesellschaft, Beriin-Siemensstadt and Erlangen, Germany, a corporation of Germany Filed Apr. 23, 1959, Ser. No. 868,384 Claims priority, application Germany Apr. 26, 1958 7 Claims. (Cl. 1136-4) My invention relates generally to thermocouples and, in a preferred aspect, to Peltier couples for the electric production of cold temperatures.

Among the materials known for use in thermocouples or Peltier couples are a group of substances of improved eicacy that possess a relatively high differential thermoelectric force together with relative low heat conductance, and hence are characterized by a small Wiedemann- Franz-Lorentz number (WFL-number). Examples of such substances are bismuth, antirnony-tellurium alloys composed of the compounds BigTeB and Sb2'1`e3. The compositions heretofore known contain more bismuth than antimony or contain both elements in substantially equal or only slightly diering proportions.

It is an object of my invention to provide thermocouple compositions and manufacturing methods by means of which a still greater thermoelectric eiiicacy than heretofore attainable can be obtained.

To this end, and in accordance with a feature of my invention, at least one member of a thermocouple, particularly a Peltier couple for the production of cold, is made of a solid solution (mixed crystal) of the compounds Bi2Te3 and Sb2Te3 with an antimony content between 24 and 36 atom percent and a bismuth content between 4 and 16 atom percent. Such solid solutions constitute a ternary system of the form:

Bi2 ysbyre,

in which y denotes a value between 1.2 and 1.8. Particularly advantageous is a composition of 28 atom percent antimony and 12 atom percent bismuth, corresponding to a value of )121.4 in the foregoing ternary formula.

The invention is based upon the surprising experimental discovery that maximum eicacies are attained if the antimony and bismuth proportions are dimensioned as stated above. The advantages of thus dimensioning the constitutents are apparent from the following tabulation which reveals, in particular, that maximum eiiicacy is achieved with a composition of 28 atom percent antimony and l2 atom percent bismuth. As a rule, the compositions according to the invention have p-type conductance. By combining such a p-conducting couple member with a second member consisting of Bi2Te3 which is n-conductively doped by an Agi addition of about 0.1% by weight, a thermocouple is obtained which, when operating as a Peltier couple, possesses a total efiicacy of 2.1--3 per C. which corresponds to a theoretically obtainable reduction in temperature of 80 C. (referring to a cold junction at 0 C.).

Sb- Bia VFL-numal()3 pv./ C. benl()8 volt2 Cri Proportion in 0.2 Atom, percent r1ree Column 1 of the table indicates the antimony content in atom percent. Column 2 indicates the bismuth component in atom percent. Column 3 indicates the diiferential thermoforce (a). Column 4 indicates the WFL- numberX l0B and column 5 indicates the efficacy (z) 103. The eicacy is defined as:

in which a denotes the dilferential thermoforce,

odenotes the electric conductance, and K denotes the heat conductance.

The efficacy value z is a direct measure for the applicability of a substance as a thermoelectric component, particularly with respect to the obtainable temperature difference in an electrothermic combination. The Value z is also a measure of the power capacity in electrothermic cooling and in thermoelectric generation of electric current.

It will be noted that the value of z is a maximum with the composition containing 28 atom percent antimony and l2 percent bismuth.

For further information reference will be made to the drawing showing in FIGS. l and 2 two respective graphs and in FIG. 3 a schematic illustration of a Peltier couple according to the invention.

Referring to FIG. 3, there is denoted by 1 a couple member consisting of a solid solution of Bi2Te3 and Sb2Te3 corresponding to the formula BiulSbMTeS and having p-type conductance. The second member 2 of the couple consists of Bi2Te3 doped with about 0.1% by weight of silver iodide (Agl) and having n-type conductance. The two members 1 and 2 are joined, by a solder or fusion joint, with three blocks 3, 4, 5 of copper. These blocks have respective channels to be joined with the pipes of coolant circulation systems. The channels of blocks 4 and 5 are to be traversed by a liquid medium for the purpose of keeping them approximately at a given temperature. The channel of the junction block 3 is to be connected into a coolant circulation line for conveying cooled liquid to the cooler, evaporator or other component to be cooled or refrigerated. As mentioned above, such a Peltier couple affords a maximum reduction of temperature of about C. Y

The thermoelectric components of a thermocouple according to the invention can be made in accordance with any of the methods known for the manufacture of thermocouple materials. Particularly advantageous is a production according to the powder-metallurgical method as generally known, for example from German Patent 836,943. One way of employing such a method for the purposes of the invention is to produce the solid solution from the compounds Bi2Te3 and Sb2Te3 by melting the respective constituents in stoichiometric proportions, then cooling and pulverizing the resulting solid solution, and thereafter processing the powdered substance in accordance with the powder-metallurgical method for shaping and sintering the thermocouple member to be produced.

According to another feature of my invention, the manufacture of the thermocouple member by a powder-metallurgical method is performed in such a manner as to adapt the thermoelectric properties to desired requirements'by suitably.dimensioningthe sintering 'data'7 namely the grain size, the amount of pressure applied to lthe pulverulent material, the sintering temperature, the sintering time and the atmosphere in which the sintering is carried out. In this manner, the defect-electron (hole)- concentrationY canY be reduced. In othery words, the

' method is preferably performed under sintering condi tions chosen for obtaining optimum charge-carrier concentration. Meant by optimum `charge-carrier'concentration is the charge-carrier concentration that results in an electronic heat conductance (nel) satisfying as close- `a current therethroughpthe thermocouple comprising twoV members of Vextrinsic'ally *conductiveV materialv electricmly connected to each other to form a junction, one of said members having p-type. conductance and being a solid solution ofBizTeS and SbzTeg having an antimony con- Y tent between aboutV 24Vl and 32 atom percent and a' bisly as feasible the following equation known from the the!V ory of the Peltier effect:

a=172 1+E ,nA/0 C.

wherein KG denotes the lattice thermal conductance.

For example, with the above-mentioned preferred com-V position containing 28 atom percent antimony and 12 atom percent bismuth, a-values of 200 to 300 ,tiv./ C.

' can be attained by varying the sintering temperature between 250 C. and 500,C., and varying the pressure force between 21a/ern.2 and 8 t./ern;2. The letter t. means a metric ton, which is 1000 kilograms. In this case the duration of the sintering processis l hour, and the grain size of the starting powder is below 60 microns. a-value of the molten material4 is 145 ,uv./ C.

,A marked improvement in the thermoelectric properties is obtained by adding up to V1% by weight of selenium to the solid solution described above.

The optimum amount of the selenium addition is about 0.6% by weight. vSuch addition, in an alloy system with the optimum y-value of 1.4,v produces' an increase in efiiciency, from about z- -2.1 `103 CF1 up to z=2l6 19"'3 @z 'i i "The ei'ect of the additions made according to the in- The vvention istdue to a reduction'in the defect-electron con- Centration which results in an increase in diterential thermoforce and hencein eiiicacy. As a result, the abovementioned optimum of thediierential thermoforce is appreached to a still further extent.

The eiect of a selenium addition upon the diierentialV thermoforce and the efficacy of the system according to the invention, relating to a composition with the optimum kiv=valiie=1.45, represented in the graphs shown fin theV drawing. FIG. 1 indicates the differential Vthermoforce ductance Von a logarithmic scale in (Sli cm.)1, and

the ordinate indicates the diiferential thermotorcey (a) in Y pv./ C. The Vdiagram indicates two curves which coincide in part. The full-line' curve VrelatesY toY measured n by pressing and sintering the powder at between about Vmuth contentbetween'about Bland 16 atoml percent and having selenium dispersed vin said solid solutionfin *an` amount of about V0.1 to 1.0% by weightrof thetotal com- K position, said member being lmade Vby preparing amelt lof said composition, causing the melt to solidify, pulverising the resultingjsolid solution, and forming said member 250 and4 500 C. Vand ata pressure between about 2 t./cm.2 and 8 t./cm.2, the vother member being Bi2Te3 nconductivity doped byaddition of AgI. Y

2. An apparatus for thermoelectric cooling,`comprising y aV thermocoupleand electric connections for passing a' current therethrough, the thermocouple comprising Ytwo i rmembers of eXtrinsically conductive material electrically connected to each other to form a junction, one of said members having p-type4 conductanceanpd being asolid solution of Bi2Te3 and Sb2Te3having an antimony content Vbetween aboutY 24 and 32" atom percent anda bi'smuth content between about Sand 16 atom percentfand having selenium dispersed in said solid Ysolutionin an amount of about 0.1 to 1.0% by weight` of the totalcornposition, said member Ybeing made by Vpreparing a melt .tent between about 24 and 32 atom percent vand aY bismuth v content between about 8 and 16 atom percent and having Y selenium dispersed in said-solid solution in an amount of l of said composition, causing thek melt to solidify,1pulveriz4 ing the resulting solidV solution, and formingsaid member by pressing and sintering the'powder at between `about f connected to each other to` form ja junction, one of said Y fr members .having p-type conductance and vbeing a solid solution of Bi2Te3 and Sb2Te3 having an antimony conabout 0.1 to'1.0% by weight of the total composition, saidV member being madeby preparing a melt of said composi- 1 tion, causing the melt tofso1idify,pulverizing the resulting values, the broken-line curve indicates theoretical values.A 1

aL-values (oc-:146, or 171, or 193) corresponding to the' measuring points shown in' FIG. 1 for'the respective se-' lenium contents of 0%, 0.5% and'1%. It is apparent that with a selenium 'addition of 0.6% by weight (a:

182) an optimum efficacy of' about 2.6 10- 3 perfa C. is

achieved.

.. the other member being n-conductive. V

solid "solution, and forming said member by, pressing'andV sintering the powder at between aboutr250"V and 500 C. and at a pressurebetween about 2 t./cm.2 and 8 `t./cm.-r,

4. The method of producing a thermocouple member,

I' which comprises thefsteps ofmelting thefcornpounds BigTearand Sb2Te3 in the proportion required for an antimonycontent between 24 and 36atom percent anda bismuth content between 4 and 16V atom percent, pulverikzing the resulting solidl solution, and forming the member by pressing and sinte'ring at a .temperaturey between` 250`C.' if and 500 Cuand a pressure between 2Vt./cm.2vjandrv8 t./cm.2.

' 5.V The Ymethod of producing a total composition, causing the melt to solidify, pulverizing Y kthe resulting mixed crystal material, andv forming they. member by pressing and sintering the powder allv a fem*VV perature between 250 and 500 C. and a pressure between 2 t./cm.2 and 8't./ cm.2.

thermocouple' member, i which comprises the steps of preparing a melt of the comil position, BizySbyTes wherein y=l,.2 to 1.8,'ladd1ng seleni- Y um Vin an amount vup(y to about l,1.0% by` weight'rof the 6. The method of producing a thermocouple member References Cited in the tile of this patent which comprises the steps of preparing a solid solution of UNITED STATES PATENTS the composition BiMSbMTeS, pulverizing said solid solution, and forming the member from the powder by sin- 2597752 Sahsbury May 20 1952 tering it at a temperature between 250 C. and 500 C. 5 2602095 Rims July 1' 1952 and a pressure between 2 t./c;m.2 and 8 t./cm.2. 762,857 Lmfenblad r- Sept' 11 1956 by sinten'ng it at a temperature between 250 C. and 500 C. and a pressure between 2 t./cm.2 and 8 t./cm.2. 

3. AN APPARATUS FOR THERMOELECTRIC COOLING, COMPRISING A THERMOCOUPLE AND ELECTRIC CONNECTIONS FORPASSING A CURRENT THERETHROUGH, THE THERMOCOUPLE COMPRISING TWO MEMBERS OF EXTRINSICALLY CONDUCTIVE MATERIAL ELECTRICALLY CONNECTED TO EACH OTHER TO FORM A JUNCTION, ONE JOF SAID MEMBERS HAVING P-TYPE CONDUCTANCE AND BEING A SOLID SOLUTION OF BI2TE3 AND SB2TE3 HAVING AN ANTIMONY CONTENT BETWEEN ABOUT 24 AND 32 ATOMS PERCENT AND A BISMUTH CONTENT BETWEEN ABOUT 8 AND 16 ATOM PERCNET AND HAVING SELENIUM DISPERSED IN SAID SOLID SOLUTION IN AN AMOUNT OF ABOUT 0.1 TO 1.0% BY WEIGHT OF LTHE TOTAL COMPOSITION, SAID MEMBER BEING MADE BY PREPARING A MELT OF SAID COMPOSITION, CAUSING THE MELT TO SOLIDIFY, JPULVERIZING THE RESULTING SOLID SOLUTION, AND FORMING SID MEMBER BY PRESSING AND SINTERING THE POWDER AT BETWEEN ABOUT 250* AND 500*C. AND AT A PRESSURE BETWEEN ABOUT 2 T./CM.2 AND 8 T./CM.2, THE OTHER MEMBER BEING N-CONDUCTIVE. 